Light-driven flow synthesis of acetic acid from methane with chemical looping

Oxidative carbonylation of methane is an appealing approach to the synthesis of acetic acid but is limited by the demand for additional reagents. Here, we report a direct synthesis of CH3COOH solely from CH4 via photochemical conversion without additional reagents. This is made possible through the construction of the PdO/Pd–WO3 heterointerface nanocomposite containing active sites for CH4 activation and C–C coupling. In situ characterizations reveal that CH4 is dissociated into methyl groups on Pd sites while oxygen from PdO is the responsible for carbonyl formation. The cascade reaction between the methyl and carbonyl groups generates an acetyl precursor which is subsequently converted to CH3COOH. Remarkably, a production rate of 1.5 mmol gPd–1 h–1 and selectivity of 91.6% toward CH3COOH is achieved in a photochemical flow reactor. This work provides insights into intermediate control via material design, and opens an avenue to conversion of CH4 to oxygenates.

EPR spectra for OH radical production over the nanocomposites in water under light irradiation, using DMPO as a trapping agent.
EPR measurement is performed to compare the concentrations of produced OH radicals over the typical WO 3 , Pd/WO 3 , PdO/Pd-WO 3 -2 and PdO/Pd-WO 3 -5 nanocomposites under light irradiation.
The ESR spectra display a 1:2:2:1 quadruplet signal, suggesting that the appropriate Pd/PdO ratio can significantly improve the generation of OH radicals 1 . However, with the further increase of PdO content, PdO will directly contact WO 3 to form PdOWO 3 interface. As a result, the lightdriven OH production is substantially suppressed due to the sluggish photo-induced charge separation at the PdOWO 3 interface. Introducing O 2 into reactants will not increase the total amount of produced liquid products but lead to CO 2 production. Among the liquid products, the CH 3 OH production and selectivity are promoted. Supplementary Fig. 22 Production rates and selectivity for oxygenates in the cyclic tests by PdO/Pd-WO 3 -2. Each cycle lasts 5 h, between which the nanocomposite is treated in air.
To perform the cyclic tests, 50 mg of nanocomposite is used in the process so that the collection of sample for regeneration treatments will be more convenient. Although the CH 4 conversion activity is related to the sample usage, our results show that the performance is approximate with the tests using 10 mg of sample in this work. Supplementary Fig. 23 The Pd loss percentage detected by ICP-MS and the CH 3 COOH production rates during cyclic tests (left side). The Pd content in material after cyclic tests is compared with the initial sample, which is detected by ICP-OES (right side). Supplementary Fig. 23, Pd 2+ can only be detected in the first round of reaction solution, corresponding to 0.16% Pd loss in the first cycle. During the measured three cyclic tests, the photochemical performance is well maintained, suggesting that the Pd atoms are stable in the PdO/Pd-WO 3 heterointerface. The total Pd content is measured by ICP-OES, showing that the Pd content after regeneration process is similar to the initial sample. It should be noted that ICP-MS is substantially more sensitive than ICP-OES. Supplementary Fig. 24 CO adsorption DRIFTS spectra of the as-prepared samples.

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As shown in Supplementary Fig. 24, the peaks at 2097 and 1983 cm -1 are ascribed to the linear-and bridge-type CO adsorption on Pd site, respectively. The new peaks arising at 2132 cm -1 over PdO/Pd-WO 3 -1 to PdO/Pd-WO 3 -3 sample are attributed to the linear CO adsorption on partially oxidized Pd 4 , demonstrating the well-formed Pd/PdO heterointerface in the samples. However, according to the report by Zorrn et al., the CO adsorption on fully oxidized Pd is weak 5 . This leads to the gradual disappearance of the peak at 2132 cm -1 over PdO/Pd-WO 3 -4 and PdO/Pd-WO 3 -5 samples. Of note, the broad peaks at 2045 cm -1 are observed, which can be assigned to bridge-type